Iron base alloys (e.g., steels) may be classified, for example, as ferritic, ferritic-austenitic (duplex), austenitic, or martensitic based on the crystal structure of the alloys. Ferritic alloys have a body-centered cubic (BCC) crystal structure. Austenitic alloys have a face-centered cubic (FCC) crystal structure. Ferritic-austenitic (duplex) alloys have a mixed microstructure of austenitic phases and ferritic phases. Ferritic alloys and austenitic alloys have stable phases that are present on an equilibrium phase diagram. Martensitic alloys have non-equilibrium, metastable phases that are not present on an equilibrium phase diagram.
Martensitic alloys may form as a result of diffusionless solid-state phase transformations in the crystal structure of parent alloys (the relative elemental compositions of martensitic alloys and phases and their parent alloys and phases are the same). The change in crystal structure is a result of a homogeneous deformation of a parent phase. For example, martensitic steels form as a result of the diffusionless solid-state phase transformation of austenitic steels from a FCC crystal structure to body-centered tetragonal (BCT) crystal structure. Martensitic phase transformations may occur in various alloys when an alloy comprising a parent phase at an elevated temperature is rapidly cooled (quenched). The cooling (quench) rate from a temperature above a martensitic transformation start temperature of an alloy to a temperature at or less than a martensitic transformation start temperature of the alloy must be sufficiently rapid to prevent solid-state diffusion and the formation of equilibrium phases.
When an alloy is rapidly cooled (quenched) from a temperature above a martensitic transformation start temperature of the alloy, a martensitic phase transformation may begin when the temperature reaches the martensitic transformation start temperature of the alloy. The extent of a martensitic phase transformation increases as the temperature of a cooling alloy decreases below the martensitic transformation start temperature. When the temperature of a cooling alloy reaches a martensitic transformation finish temperature, the crystal structure of the alloy may have entirely transformed from the parent phase to a non-equilibrium, metastable martensitic phase. If a cooling alloy is held at an intermediate temperature between the martensitic transformation start temperature and the martensitic transformation finish temperature, the extent of the martensitic phase transformation does not change with time.